Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2000-09-11
2003-09-30
Anderson, Bruce (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200, C250S397000, C250S310000
Reexamination Certificate
active
06627903
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle, to a suitable substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to calibrations, as performed with a charged-particle-beam microlithography apparatus, directed to achieving accurate alignment of the reticle with the substrate.
BACKGROUND OF THE INVENTION
In charged-particle-beam (CPB) microlithography (i.e., microlithography performed using a charged particle beam such as an electron beam or ion beam), as in optical microlithography (i.e., microlithography performed using visible or ultraviolet light), obtaining accurate alignment between the reticle and the substrate is extremely important. Current microlithography apparatus include sophisticated devices for determining reticle-substrate alignment. In a CPB microlithography apparatus as currently available, these alignments involve impinging a charged particle beam on a mark on the substrate or substrate stage and detecting charged particles (e.g., electrons) backscattered from the mark.
In various techniques for determining reticle-substrate alignment using a charged particle beam, it is important to be able to determine and calibrate the angle of incidence of the beam on the substrate (specimen) surface. A conventional method for calibrating the angle of incidence of an electron beam on a specimen surface is shown in FIG.
3
. In the figure, an electron beam EB propagating along an axis AX is deflected by a deflector
70
to impinge on a surface
100
of a specimen (e.g., semiconductor wafer). The surface
100
includes an alignment mark
101
. When electrons of the beam EB impinge on the alignment mark
101
, backscattered electrons are produced that propagate to and are detected by a backscattered-electron (BSE) detector
72
situated upstream of the surface
100
. Typically, the deflector
70
deflects the beam EB in a scanning manner over the mark
101
.
To perform the conventional calibration method, a calibration specimen (having a surface
100
bearing an alignment mark
101
) is mounted on a specimen stage. The specimen stage is positioned such that the surface
100
is at a desired first axial position (first “height”). With the specimen positioned in this manner, the electron beam EB irradiates and is scanned across the specimen. The position of the alignment mark
101
is determined from a BSE signal waveform produced by the BSE detector
72
, based on backscattered electrons propagating from the alignment mark
101
.
After obtaining the BSE signal waveform from the alignment mark
101
, the specimen table is moved axially (in the Z-direction) to place the surface
100
at a second “height” (the surface at the second height is denoted
100
′, and the alignment mark on the surface at the second height is denoted
101
′). The difference between the first height and second height is denoted &Dgr;Z. With the specimen positioned at the second height, the electron beam EB irradiates and is scanned across the specimen. The position of the alignment mark
101
′ is determined in the same manner as described above with respect to the alignment mark
101
.
Referring further to
FIG. 3
, a respective angle &thgr; at which the electron beam EB is incident to the specimen surface
100
,
100
′ is found by determining &Dgr;X (difference in X-axis position of the alignment mark
101
′ relative to the X-axis position of the alignment mark
101
) and &Dgr;Z (change in elevation of the specimen stage). Normally, whenever the deflection angle of the electron beam relative to the axis AX is zero degrees, the angle of incidence &thgr; of the electron beam on the specimen surface should be zero degrees. Consequently, an electron-optical system (through which the electron beam passes) or the specimen stage is adjusted such that the angle &thgr; is zero degrees.
In the conventional method described above, the accuracy with which the angle &thgr; is determined is a function of the magnitude of Z-direction movement (&Dgr;Z) of the specimen stage and the accuracy with which the alignment mark is detected. For example, if the Z-direction movement &Dgr;Z is 10 &mgr;m and the alignment-mark detection accuracy is 0.1 &mgr;m, then the approximate accuracy (&Dgr;&thgr;) with which the angle &thgr; can be detected is determined as follows:
&Dgr;&thgr;=arctan(0.1 &mgr;m/10 &mgr;m)=10 mrad
(10 mrad is a common value for this accuracy). If the Z-direction movement (&Dgr;Z) is 100 &mgr;m and the alignment-mark detection accuracy is 10 nm, then the accuracy with which the angle &thgr; should be detected is approximately 100 &mgr;rad. To obtain this accuracy, an angle-detection accuracy of approximately 10 &mgr;rad is required. Unfortunately, with a conventional electron-beam microlithography apparatus, such accuracy is difficult to achieve.
Generally speaking, it is difficult to improve the movement in the Z-direction and the alignment-mark detection accuracy more than the respective amounts discussed above. It also is difficult to obtain an alignment-mark detection accuracy of less than 100 &mgr;rad. In addition, whenever the height of the specimen stage is changed, accompanying lateral shifts also are encountered frequently. Hence, conventional methods for performing reticle-substrate alignment have factors that contribute significantly to degradations in detection accuracy.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide, inter alia, reticle-substrate calibration methods (as used with a charged-particle-beam (CPB) microlithography apparatus) that achieve high-accuracy calibration results using a simple procedure. Another object of the present invention is to provide microelectronic-device manufacturing methods by which high-accuracy patterns can be formed using a CPB microlithography apparatus calibrated according to the invention.
To such ends, and according to a first aspect of the invention, methods are provided (in the context of performing a CPB microlithography of a specimen using a CPB microlithography apparatus) for calibrating the CPB microlithography apparatus. In a representative embodiment of such a method, a specimen is provided presenting a surface having a crystalline structure. A charged particle beam (e.g., electron beam) is irradiated onto an area of the surface by scanning the beam in X- and Y-dimensions. While monitoring X- and Y-dimension beam-scanning coordinates of the scanned area, backscattered charged particles produced by the area being irradiated are detected. A corresponding backscattered-particle electrical signal is produced. The signal contains signal-amplitude data as a function of the X- and Y-dimension beam-scanning coordinates. The data in the signal are processed to produce a map pattern of signal amplitude as a function of X- and Y-dimension beam-scanning coordinates. Also produced are data regarding whether a center of the map pattern is aligned with an origin of X- and Y-dimension beam-scanning axes. If the center of the map pattern is not aligned with the origin of X- and Y-dimension beam-scanning axes, then an adjustment is performed to achieve such alignment so as to calibrate the apparatus.
In the foregoing method, the data-processing step can include calibrating an angle of incidence of the charged particle beam on the surface. Also, the step of providing a specimen can include providing a monocrystalline silicon specimen of which the presented surface has a 111 crystal-lattice structure.
In another representative embodiment, a specimen (presenting a surface having a given crystal-orientation plane) is mounted on a specimen table of the CPB microlithography apparat
Anderson Bruce
Klarquist & Sparkman, LLP
Nikon Corporation
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